1. Field of the Invention
This invention relates to a method for producing a film made of a thermoplastic material, wherein a melt of a plastic material exits from a blow head supplied by at least one extruder, while forming a film bubble, and then the film bubble passes through a calibrating arrangement, in which it is calibrated to a predeterminable diameter, as well as afterwards is flattened into a film web in a flattening device, and then is rolled up, for example. This invention also relates to a device for producing a film made of a thermoplastic material, having at least one extruder with a blow head for the emergence of a film bubble from the plastic material, a calibrating arrangement acting on the exterior of the film bubble, and a flattening device for the film bubble arranged downstream of the calibrating arrangement, wherein the calibrating arrangement and/or the flattening device have film guidance elements, which can be charged with an airflow, and wherein air outlet openings for the exit of the supplied air flow are provided in the area of the surface of the film guidance elements facing the film bubble.
2. Discussion of Related Art
Production processes for the manufacture of films, also called blown films, from a thermoplastic material are known.
The thermoplastic, such as a molten state as the plastic material emerges from the customarily ring-shaped opening of the blow head and is immediately blown up in to a film bubble, hardens while cooling below the solidification temperature. The film bubble is also guided through a calibrating arrangement and its diameter is calibrated, and then it is flattened by a flattening device into a double film and wound up thereafter.
In this case the calibrating arrangement is customarily embodied in the shape of a so-called calibrating basket, which surrounds the exterior of the film bubble as a cylinder and includes a plurality of calibrating arms, which are in contact on the outside with the circumference of the film bubble in order to fix its diameter and to calibrate it. For example, the calibrating arms can have a plurality of rotatable plastic rollers, which roll off on the surface of the film bubble and simultaneously calibrate it. Embodiments with rotating brushes or the like are also known. Customarily, the calibrating arms of such a calibrating basket are adjustably held, so that various diameters of the film bubble can be set and calibrated.
These known methods are distinguished because during its passage through the calibrating arrangement, the film bubble contacts the calibrating device, for example the calibrating arms and the plastic rollers fastened thereon, because direct contact with them occurs. However, this contact of the calibrating arrangement with the film, which has not yet been completely cooled and solidified in the area of the calibrating arrangement, is disadvantageous, for example when producing highly transparent or tacky film, because respective running marks can form on the film with this contact, which then has a negative effect on the visual appearance of the finished film. In case of tacky films, a blockage of the calibrating arrangement might even occur, which afterwards results in damage to the film bubble and a corresponding involuntary production stoppage.
The above remarks apply in the same way to the flattening device arranged downstream of the calibrating arrangement, in which the film, with a diameter that has already been calibrated, is flattened into a film web.
Therefore a calibration and/or flattening of the film bubble without a possible contact with it can be of considerable advantage in particular applications. European Patent Reference EP 0 273 739 A1 teaches guiding the film bubble in a contactless manner between strong air flows acting on the inside and outside of the film bubble to prevent contact friction causing the surface blemishes of the film bubble. However, the cost outlay for an apparatus with high-output blowers, air conduits, as well as an appropriate control technology, is very great, and there is a problem with too strong air flows causing undesirable temperature effects on the film bubble might occur, which negatively affect the quality of the film obtained.
Furthermore, a device for contactless guidance or treatment of a running web of material, for example a plastic film, is taught by European Patent Reference EP 1 144 292 B1, wherein a gaseous fluid for guiding the film exits from a chamber, wherein the gas-permeable wall is made of a metal-containing plastic material with open pores. However, the known device is employed for guiding a web of already great stability through a coating device.